1. Field of the Invention
The invention relates to a process for the preparation of a melamine condensation product, and more particularly, to tho preparation of a melamine condensation product that is free of halogens and imparts flame-retardancy.
2. Description of Related Art
Melamine forms melamine condensation products when heated under certain reaction conditions. Ammonia is evolved in the reaction. Similarly, melamine salts form condensation products when heated. Melamine condensation products include melam, melone, and melam, as well as salts thereof. Generally, melam (C.sub.6 H.sub.9 N.sub.11) forms upon heating melamine and/or melamine salts below 315.degree. C. and is a by-product of melamine synthesis. For purposes of the present invention, a melamine condensation product encompasses a product that results from self-condensation of melamine or a melamine salt with the evolution of ammonia. Hence, for example, a melamine condensation product is not the product from the condensation reaction between melamine and, for example, formaldehyde to yield amino resins.
Specific processes relating to the preparation of melam via condensation are described in, for example, V. A. Gal'perin et al., Zhurnal Organicheskoi Khimii, Vol. 7, No. 11, pp. 2431-2432 (November 1971) and Gavrilova et al., Zhurnal Organicheskoi Khimii, Vol. 13, No. 3, pp. 669-670 (March 1977). In laboratory scale experiments, melamine was converted to a salt of melam with use of a ZnCl.sub.2 condensation agent at a temperature between 290.degree. C. and 320.degree. C. Specifically, 3.3 grams of melamine was mixed with 1.7 grams of anhydrous zinc chloride and heated for 1 hour at a temperature of 290.degree. C. in an open tube. The open tube is a so-called stationary system. The product was washed with boiling methanol to remove residual zinc chloride. It was then washed with diluted hydrochloric acid to remove residual melamine. The zinc salt of melam thus obtained contained 87.2 wt. % of melam and 11.8 wt. % of zinc. In an additional treatment to prepare melam from the melam zinc salt, the zinc salt of melam was boiled for 5-7 minutes in a 1% solution of sodium hydroxide. Next, the solution was filtered off and cooled. The precipitate was washed with hot water and dried for 2 hours at a temperature of 170.degree. C. The yield of the product was 2.7 grams.
The percentages of zinc and chlorine in the melam product after this additional treatment were not reported in this reference. In later work, however, the melam product of this reaction was found to have a residual content of 3.5 wt. % of zinc and 1.6 wt. % of chloride. In other words, significant amounts of zinc chloride remain in the melam, even after attempts to remove it.
The presence of zinc and chloride impurities is a significant drawback. Both ions are difficult to wash out. Furthermore, washing zinc and chloride out of the product can result in a high degree of hydrolysis of the melam.
Alternatively, melamine can also be converted to a condensation product like melam on a laboratory scale (e.g. milligram or gram scale) in the presence of inorganic acids as condendation agents. Inorganic acids include HCl, HBr, sulfuric acid, phosphoric acid, and mixtures thereof. The ammonia or melamine salts of these acids can also be used. However, there are drawbacks to these alternative small scale syntheses. These include, among others, a need for high reaction temperature, low yields, a high degree of melamine sublimation, formation of by-products, and halogen residues in the end product. These drawbacks render these alternatives unsuitable for commercial scale (e.g. kilogram- or tonnage scale) production of a condensation product of melamine.